3. Introduction
UNIX is a portable, multi-tasking and multi-user operating
system.
– Portable: runs on many different hardware architectures (Intel
x86 and IA-64, Alpha, MIPS, HP PA-RISC, PowerPC, IBM
S/390, SPARC, Motorola 680x0, etc.).
– Preemptive multi-tasking: several programs can run at the
same time (time slices, interrupts, and task switching).
– Multi-user: many users can share the computer system at the
same time.
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4. Other Features…
• Uses a simple, uniform file model which includes devices and
access to other services in a flexible, hierarchical file system.
• Written in a high-level language (“C”) making it easy to
read, understand, change and port.
• The command prompt is a simple user process, the Unix shell,
which is also a convenient job programming language.
• Includes support for regular expressions which are convenient
for complex searching.
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5. History of UNIX
• 1964 joint project between AT&T Bell Labs, GE, and MIT to
develop a new OS.
• Goal : develop an OS that could provide computational
power, data storage and the ability to share data among
multiple users.
• Result: Multiplexed Information & Computer Service -
MULTICS.
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6. • 1969 Bell Labs withdraws from group.
• Two Bell Lab scientists, Ken Thompson and Dennis Ritchie,
continue research. They were still left without a “Convenient
interactive computing service”*.
• At the same time Ken Thompson wrote a game “space
travel” in Fortran to run on GECOS OS.
• The spaceship was hard to control and it was expensive to
run. He was told to get the game off his work computer.
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7. • Thompson ported the game to a little used PDP-7 computer.
• Unics (later Unix) was born as a pun on Multics.
• Dennis Ritchie developed “B” . Then wrote “C” a compiled
language.
• In 1973 entire OS ported to “C”.
• 1991 Linux 0.02 is first released to the public.
• 1994 Linux 1.0 is released.
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8. Memory Management
UNIX is machine independent so its memory management
scheme will vary from one system to next.
Early versions of UNIX used variable partitioning with no
virtual memory scheme.
Current implementations of UNIX make use of paged virtual
memory.
There are two memory management schemes :
Paging System
Kernal Memory Allocator
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10. Memory Management policies
• Swapping
– Easy to implement
– Less system overhead
• Demand Paging
– Greater Flexibility
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11. Swapping
• The process of moving some pages out of main memory and
moving others in, is called swapping.
• A page fault occurs when the CPU tries to access a page that
is not in main memory, thus forcing the CPU to wait for the
page to be swapped in.
• Since moving data to and from disks takes a significant
amount of time, the goal of the memory manager is to
minimize the number of page faults.
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12. • Swap Space - Disk memory used to hold data that is not in
Real or File System memory. Swap space is most efficient
when it is on a separate disk or partition, but sometimes it is
just a large file in the File System.
• Allocation of both main memory and swap space is done
first-fit.
• A page fault occurs when the CPU tries to access a page
that is not in main memory, thus forcing the CPU to wait for
the page to be swapped in.
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13. • Since moving data to and from disks takes a significant
amount of time, the goal of the memory manager is to
minimize the number of page faults.
• When the size of a process' memory image increases (due to
either stack expansion or data expansion), a new piece of
memory big enough for the whole image is allocated.
• If no single piece of main memory is large enough, the
process is swapped out such that it will be swapped back in
with the new size.
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14. • Decisions regarding which processes to swap in or swap out
are made by the scheduler process (also known as the
swapper).
• A process is more likely to be swapped out if it is idle or has
been in main memory for a long time, or is large ; if no
obvious candidates are found, other processes are picked by
age.
• A process is more likely to be swapped in if its has been
swapped out a long time, or is small.
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16. Allocating Swap Space
Address Unit
Allocate 100 unit
1 10000 101 9900
Map
Allocate 50 unit
Allocate 100 unit
251 9850 151 9750
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17. Freeing Swap Space
Address Unit
50 unit free at 101
251 9750 101 50
Map 251 9750
Case 1: Free resources fill a hole,
but not contiguous to any resources in the map
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18. Freeing Swap Space
Address Unit
50 unit free at 101 101 50
251 9750
251 9750
Map
100 unit free at 1
1 150
Case 2: Free resources fill a hole,
251 9750
and immediately precedes an entry in the map
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19. Freeing Swap Space
Address Unit 101 50
50 unit free at 101
251 9750 251 9750
Map
100 unit free at 1
1 150 Allocate 200 unit 1 150
451 9550 251 9750
300 unit free at 151 Case 3: Free resources fill a
hole, and completely fills the
1 10000 gap between entries in the
map 19
20. Demand Paging
• Demand paging to unix with BSD(Berkley system) which
transferred memory pages instead of process to and from a
secondary device.
• When a process needs a page and the page is not there, a
page fault to the kernel occurs a frame of main memory is
allocated, and the process is loaded into the frame by the
kernel.
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21. Page Table
Page Frame Age Copy on Modify Reference Valid Protect
Number write
• frame # contains the physical frame where the virtual page
is stored
• age is processor dependant, and is meant to maintain how
long it has been since the page was accessed.
• Copy on Write store the copy on write bit, which is used in
UNIX systems to, among other things, render fork efficient.
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22. • Dirty is a single bit that indicates whether a page has been
modified since last swapped in (the opposite of dirty is clean,
and a clean page need not be written out to disk if
swapped).
• Ref contains the usage information necessary for a CLOCK-
style algorithm.
• Valid is the standard UNIX jargon for resident. A valid page
is in main memory, an invalid one is swapped out.
• Protect contains the permission information for the page
and is also hardware dependant.
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23. Disk block descriptor
Swap device Number Device Block No. Type of storage
• The disk block descriptor contains the information mapping
a virtual page to a spot on disk.
• The OS maintains a table of descriptors for each process.
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24. • Device # is basically a pointer to the disk that this page was
swapped to.
• Block # is the actual block that the page is stored on. This is
why most UNIX systems prefer to have a separate swap
partition, so that the block size can be set to the page size.
• Type specifies whether the page is new or pre-existing. This
lets the OS know if it has to clear the frame first.
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25. Page frame data table
Page State Reference Logical Block Pf data
Count device number pointer
• The page frame data table holds information about each
physical frame of memory (indexed by frame number).
• This table is of primary importance for the replacement
algorithm.
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26. • Page state indicates whether or not the frame is available or
has an associated page (i.e. whether its been allocated to a
process or not).
• Ref. Count holds the number of processes that refer to this
page (remember, processes can share the same physical
page).
• Logical device contains the device number of the disk that
holds a physical copy of the page.
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27. • Block # holds the block number on that disk where the
page data is located.
• Pfdata pointer is a pointer field that is used to thread a
singly-linked list of free frames through the frame table. If
the page is free, this points to the next free page (useful for
free list-style allocation).
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28. Swap use table
Reference Page/storage
count unit number
• Reference Count : Number of page table entries that point
to a page on the swap device.
• Page/storage unit number : Page identifier on storage unit
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29. Page Replacement Algorithm
• The Page frame data table is used for page replacement.
• All of the available frames are linked together in a list of free
frames available for bringing in pages.
• The two-handed clock algorithm uses the reference bit in the
page table entry for each page in memory that is eligible
(not locked) to be swapped out.
• This bit is set to 0 when when the page is first brought in and
set to 1 when the page is refernced for a read or write .
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31. • Two parameters determine the operation of the algorithm:
– Scanrate: The rate at which the two hands scan through the
page list, in pages per second.
– Handspread: The gap between fronthand and backhand
• These two parameters have default values set at boot time
based on the amount of physical memory.
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32. • The parameter varies linearly between the values slowscan
and fastscan as the amount of free memory varies between
the values lotsfree and minfree.
• The handspread parameter determines the gap between the
fronthand and the backhand and therefore, together with
scanrate,determines the window of oppurtunity to use a
page before it is swapped out due to lack of use.
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33. Kernal Memory Allocator
• Kernel memory allocator: provides buffers of memory to
various kernel subsytems.
Evolution Criteria :
– must be space-efficient i.e. minimize wastage.
– Can be measured by utilization factor
– must be fast
– must have a simple programming interface
– should not force to free the entire allocated area all at once
– must guard against the wastage of memory
– must be able to interact with the paging system
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34. Advantages
• easy to implement
• not restricted to memory allocation
• no wastage of space
• can release any part of the region
• allows reuse of memory by coalescing
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35. References
Books :
*********
1. Os Internals & design
--- William Stallings
2. The design of the unix operating system
--- Maurice J. Bach prentice hall
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